Solid state carbon dioxide sensor

ABSTRACT

A solid state, carbon dioxide (CO 2 ) sensor configured for sensitive detection of CO 2  in both dry and moist conditions. The CO 2  sensor utilizes a composite sensing material that detects CO 2  in the range of 100 ppm to 10,000 ppm. The sensing material is composed of O-MWCNTs and a metal oxide functionalizing agent, such as iron oxide (Fe 2 O 3 ) nanoparticles. The material has an inherent resistance and conductivity that is chemically modulated as the level of CO 2  increases. The CO 2  gas molecules that are absorbed into the carbon nanotube composites cause charge-transfer and changes in the conductive pathway causes changes in conductivity of the composite sensing material. This change in conductivity provides a specificity and sensitivity for CO 2  detection. The CO 2  sensor can be easily integrated into existing electronic circuitry and hardware configurations, including the hardware of a mobile computing device, such as a smart phone or tablet device.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/936,379, filed Nov. 15, 2019, the contents of which are incorporatedby reference as if fully set forth herewith.

ORIGIN OF INVENTION

The invention described herein was made in the performance of work undera NASA contract and by an employee of the United States Government andis subject to the provisions of Public Law 96-517 (35 U.S.C. § 202) andmay be manufactured and used by or for the Government for governmentalpurposes without the payment of any royalties thereon or therefore. Inaccordance with 35 U.S.C. § 202, the contractor has elected not toretain title.

TECHNICAL FIELD

The present invention generally relates to carbon dioxide sensors.

BACKGROUND OF THE INVENTION

Monitoring carbon dioxide (CO₂) levels is critical because carbondioxide displaces oxygen. The displacement of oxygen by carbon dioxidecauses negative health effects. There is a great demand for effectivecarbon dioxide monitoring devices for a variety of applications such asmonitoring global warming, measuring air quality, healthcare, mining andthe food industry. In space applications, CO₂ detectors are very crucialat the International Space Station (ISS) for monitoring CO₂ levels inthe crews' cabins and during CO₂ sequestration processes to ensure thatCO₂ is scrubbed. It is also necessary to monitor CO₂ in a space suit,particularly within the astronauts' helmets.

Carbon dioxide is harmful at higher concentrations due to its ability todisplace oxygen in high concentrations. There is about 0.04% (400 ppm)CO₂ present in normal air. Such a concentration is basically harmless.However, once the CO₂ concentration of inhaled air exceeds 1.0%, the CO₂begins to cause harmful effects on the human body. Headaches typicallystart within a few hours of CO₂ levels of 2.0%-3.0%. At CO₂ levelsbetween 4.0%-5.0%, a person begins to experience increased bloodpressure and breathing issues. At CO₂ levels above 5.0%, a person canbecome incapacitated. Coma and possibly death can occur within minutesof CO₂ levels reaching 17%. The Occupational Safety and HealthAdministration (OSHA) set forth a maximum permissible exposure limit forCO₂ at 0.5% (5000 ppm) air composition in an eight-hour work day.

CO₂ toxicity is extremely dangerous for workers in confinedenvironments. Conventional nondispersive infrared (NDIR) sensors havebeen used for CO₂ detection. An NDIR sensor works by comparing theamount of infrared light absorbed by the CO₂ with the amount of CO₂emitted to correlate its ratio to CO₂ concentration. The accuracybecomes problematic when the gas absorption lines begin to broaden dueto local effects of humidity. Specifically, at higher pressures,temperatures and humidity levels, radiating and collision time increasethereby causing the absorption lines to be broaden. Other conventionalCO₂ sensors include silicon nanowires, Sn₂O₃, microspheres, and polymernanofilms. However, these conventional sensors have exhibitedshortcomings in range, response time, accuracy at different temperaturesand pressures, power consumption and size. Furthermore, many of theseconventional sensors have difficulty operating at room temperature andalso have poor responses in static diffusion mode conditions.

Accurate, quick-response and low power CO₂ sensors are needed for manyapplications, such as measuring indoor air quality, modifiedatmospheres, stowaway detection, cellar and gas stores, marine vessels,greenhouses, monitoring landfill gas, confined spaces, cryogenics,ventilation management, mining and rebreathers (SCUBA). Accurate,quick-response and low power CO₂ sensors are also needed in spaceapplications, such as space medicine, in-situ resource utilization ofCO₂ on Mars, CO₂ removal in life support systems on space vessels, andmonitoring the air quality in the crew's cabin. In view of the toxicitycaused by inhalation of high levels of CO₂, what is needed is alightweight, low cost, solid state CO₂ sensor that quickly providesaccurate readings at different temperatures and pressures, provides forin-situ monitoring, operates at room temperature, consumes extremely lowpower (in the order of microwatts) and has a relatively small size.

SUMMARY OF THE INVENTION

Described herein are various exemplary embodiments of a solid state,carbon dioxide (CO₂) sensor configured for sensitive detection of CO₂having a concentration within the range of about 100 ppm and 10,000 ppmin both dry conditions (e.g., ambient air) and high humidity conditions(e.g., >80% relative humidity). The solid state, carbon dioxide (CO₂)sensor achieves detection of high concentrations of CO₂ withoutsaturation and in both dynamic flow mode and static diffusion modeconditions. The composite sensing material comprises oxidizedmulti-walled carbon nanotubes (O-MWCNT) and a metal oxide. In anexemplary embodiment, the composite sensing material comprises O-MWCNTand iron oxide (Fe₂O₃) nanoparticles. The composite sensing material hasan inherent resistance and corresponding conductivity that is chemicallymodulated as the level of CO₂ increases. The CO₂ gas molecules absorbedinto the carbon nanotube composites cause charge-transfer and changes inthe conductive pathway such that the conductivity of the compositesensing material is changed. This change in conductivity provides aspecificity and sensitivity for the CO₂ detection. The solid state CO₂sensor of the present invention provides many benefits and advantagessuch a relatively light weight, small footprint and low powerconsumption. The solid state CO₂ sensor of the present invention can beeasily integrated into existing electronic circuitry and hardwareconfigurations. The solid state CO₂ sensor is well suited for automatedmanufacturing using robotics and software controlled operations. Thesolid state CO₂ sensor does not utilize consumable components ormaterials and does not require calibration as often as conventional CO₂sensors. Since the solid state CO₂ sensor of the present invention canbe easily integrated into existing programmable electronic systems orhardware systems, the calibration of the CO₂ sensor can be automated.

In some embodiments, the present invention is directed to a solid statesensor for sensing carbon dioxide (CO₂) comprising a substrate,electrodes disposed on the substrate and spaced apart by a predetermineddistance, and a sensing material disposed over the electrodes. Thesensing material comprises an oxidized carbon nanostructure having afunctionalizing agent comprising iron oxide (Fe₂O₃). In an exemplaryembodiment, the oxidized carbon nanostructure comprises O-MWCNTs. Insome embodiments, the weight percentage of iron oxide (Fe₂O₃) in thesensing material is between about 3.25% and 3.75%. In an exemplaryembodiment, the weight percentage of iron oxide (Fe₂O₃) in the sensingmaterial layer is about 3.5%. The solid state (CO₂) sensor furthercomprises a plurality of electrically conductive output terminals. Eachoutput terminal is electrically coupled to a corresponding electrode.

In some embodiments, the present invention is directed to a solid statesensor for sensing carbon dioxide (CO₂) comprising a substrate, a pairof interdigitated electrodes disposed on the substrate and spaced apartby a predetermined distance and sensing material disposed over theinterdigitated electrodes. The sensing material comprises O-MWCNTshaving a functionalizing agent comprising iron oxide (Fe₂O₃). In someembodiments, the weight percentage of iron oxide (Fe₂O₃) in the sensingmaterial is between about 3.25% and 3.75%. In an exemplary embodiment,the weight percentage of iron oxide (Fe₂O₃) in the sensing material isabout 3.5%. The solid state (CO₂) sensor further comprises a pair ofelectrically conductive output terminals. Each output terminal iselectrically coupled to a corresponding interdigitated electrode. Thesolid state (CO₂) sensor further comprises a circuit electricallyconnected between the pair of electrically conductive output terminalsto provide electrical power to the pair of interdigitated electrodes andto detect and measure the current flow through the pair ofinterdigitated electrodes.

In some embodiments, the present invention is directed to a method offabricating a solid state sensor for sensing carbon dioxide, comprisingproviding a substrate, disposing a pair of electrodes on the substrate,providing a predetermined amount of multi-walled carbon nanotubes(MWCNTs), and oxidizing the MWCNTs with a mixture of acids at a firstpredetermined temperature and a first predetermined amount of time toproduce O-MWCNTs. The method further comprises diluting, decanting andcentrifuging the O-MWCNTs, rinsing the O-MWCNTs in water to producepurified O-MWCNTs and drying the purified O-MWCNTs at a secondpredetermined temperature for a second predetermined amount of time toproduce purified O-MWCNTs in dry powder form. The method furthercomprises forming a solution consisting of water, the purified O-MWCNTsand iron oxide (Fe₂O₃) nanoparticles so as to produce a uniformsuspension, depositing the uniform suspension onto the electrodes, andthereafter, baking the substrate so as to transform the uniformsuspension into a dry (CO₂) sensing material.

In some embodiments, the present invention is directed to an internalcarbon dioxide (CO₂) sensor that comprises a substrate, electrodesdisposed on the substrate and spaced apart by a predetermined distanceand a sensing material disposed over the electrodes such that the sensoris electrically, electronically, or communicatively coupled to a mobilecomputing device, such as a smart phone, tablet, or other mobilecomputing device running a mobile operating system, such as Apple,Inc.'s iOS or Google, Inc.'s Android operating system. The sensingmaterial comprises an oxidized carbon nanostructure having afunctionalizing agent comprising iron oxide (Fe₂O₃). In an exemplaryembodiment, the oxidized carbon nanostructure comprises O-MWCNTs. Theinternal carbon dioxide sensor further comprises a plurality ofelectrically conductive output terminals. Each output terminal iselectrically coupled to a corresponding electrode.

Programmable internal electronic circuitry in the mobile computingdevice includes data acquisition circuitry that is coupled to theelectrically conductive output terminals of the carbon dioxide sensorand configured for detecting and measuring the electrical current flowthrough the electrodes. The mobile computing device is programmed withan application program (i.e., mobile app) that when executed, isconfigured to determine if the detected current flow represents aconcentration of carbon dioxide that exceeds a predeterminedconcentration of carbon dioxide, and alerts a user if it is determinedthat the concentration of carbon dioxide exceeds the predeterminedconcentration of carbon dioxide. The programmable internal electroniccircuitry also may also run the operating system of the mobile computingdevice, including executing and implementing other smart phonefunctionality. In other words, the sensor may be integrated into or withthe internal electronic circuitry or other hardware components of asmart phone or other mobile computing device. In some embodiments, theweight percentage of iron oxide (Fe₂O₃) in the sensing material isbetween about 3.25% and 3.75%. In an exemplary embodiment, the weightpercentage of iron oxide (Fe₂O₃) in the sensing material layer is about3.5%.

Certain features and advantages of the present invention have beengenerally described in this summary section. However, additionalfeatures, advantages and embodiments are presented herein or will beapparent to one of ordinary skill of the art in view of the drawings,specification and claims hereof. Accordingly, it should be understoodthat the scope of the invention shall not be limited by the particularembodiments disclosed in this summary section.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional view of a solid state carbon dioxide sensorin accordance with some embodiments of the present invention;

FIG. 1B is a perspective view of the sensing material of the solid statecarbon dioxide sensor;

FIG. 1C is a perspective view of a substrate and interdigitatedelectrodes of the solid state carbon dioxide sensor;

FIG. 1D is a flow diagram of a process for fabricating the solid statecarbon dioxide sensor;

FIG. 1E is a block diagram of a system used to analyze the sensitivityof the solid state carbon dioxide sensor;

FIG. 2 is a plot of the responses of the solid state carbon dioxidesensor to increasing concentrations of CO₂ gas;

FIG. 3 is a FE-SEM image of the O-MWCNT/iron oxide composite sensingmaterial of the solid state carbon dioxide sensor;

FIG. 4 shows the response curves for the solid state carbon dioxidesensor of the present invention and a conventional oxidized MWCNT-basedsensor for various concentrations of CO₂;

FIG. 5 is a plot of the responses of the solid state carbon dioxidesensor of the present invention for multiple exposures to a CO₂concentration of 4000 ppm; and

FIG. 6 is an exemplary schematic diagram of a smart phone incorporatingthe solid state carbon dioxide sensor of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention may be understood more readily by reference to thefollowing detailed description taken in connection with the accompanyingfigures and examples, which form part of this disclosure. It is to beunderstood that this invention is not limited to the specific devices,methods, conditions or parameters described and/or shown herein, andthat the terminology used herein is for the purpose of describingparticular embodiments by way of example only and is not intended to belimiting of the claimed invention. As used herein, the terms“comprises,” “comprising,” “includes,” “including,” “has,” “having” orany other variation thereof, are intended to cover a non-exclusiveinclusion. For example, a process, method, article or apparatus thatcomprises a list of elements is not necessarily limited to only thoseelements, but may include other elements not expressly listed orinherent to such process, method, article or apparatus. Also, as used inthe specification including the appended claims, the singular forms “a,”“an,” and “the” include the plural. Any numerical parameters set forthin the specification and attached claims are approximations (forexample, by using the term “about”) that may vary depending upon thedesired properties sought to be obtained by the present invention. Allranges of numerical values are inclusive.

It is to be appreciated that certain features of the invention whichare, for clarity, described herein in the context of separateembodiments, may also be provided in combination in a single embodiment.Conversely, various features of the invention that are, for brevity,described in the context of a single embodiment, may also be providedseparately or in any sub-combination.

As used herein, the abbreviation “MWCNT” shall refer to “multi-walledcarbon nanotube” and the abbreviation “MWCNTs” shall refer to“multi-walled carbon nanotubes”.

As used herein, the abbreviation “O-MWCNT” shall refer to “oxidizedmulti-walled carbon nanotube” and the abbreviation “O-MWCNTs” shallrefer to “oxidized multi-walled carbon nanotubes”.

In accordance with one embodiment of the present invention, thecomponents, process steps, and/or data structures may be implementedusing various types of operating systems (OS), computing platforms,firmware, computer programs, computer languages, and/or general-purposemachines. The method can be run as a programmed process running onprocessing circuitry. The processing circuitry can take the form ofnumerous combinations of processors and operating systems, or astand-alone device. The process can be implemented as instructionsexecuted by such hardware, hardware alone, or any combination thereof.The software may be stored on a program storage device readable by amachine.

In addition, those of ordinary skill in the art will recognize thatdevices of a less general purpose nature, such as hardwired devices,field programmable logic devices (FPLDs), including field programmablegate arrays (FPGAs) and complex programmable logic devices (CPLDs),application specific integrated circuits (ASICs), or the like, may alsobe used without departing from the scope and spirit of the inventiveconcepts disclosed herein.

In accordance with one embodiment of the present invention, the methodmay be implemented on a data processing computer such as mobilecomputing device, a personal computer, workstation computer, mainframecomputer, or high performance server running an OS such as Solaris®available from Oracle Corporation of Redwood City, Calif., Microsoft®Windows®, available from Microsoft Corporation of Redmond, Wash.,various versions of the Unix operating system such as Linux availablefrom a number of vendors, various embedded operating systems, or variousmobile operating systems. The method may also be implemented on amultiple-processor system, or in a computing environment includingvarious peripherals such as input devices, output devices, displays,pointing devices, memories, storage devices, media interfaces fortransferring data to and from the processor(s), and the like.

The features, structures, or characteristics of the invention describedthroughout this specification may be combined in any suitable manner inone or more embodiments. For example, reference throughout thisspecification to “certain embodiments,” “some embodiments,” or similarlanguage means that a particular feature, structure, or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present invention. Thus, appearances of the phrases“in certain embodiments,” “in some embodiment,” “in other embodiments,”or similar language throughout this specification do not necessarily allrefer to the same group of embodiments and the described features,structures, or characteristics may be combined in any suitable manner inone or more embodiments.

It should be noted that reference throughout this specification tofeatures, advantages, or similar language does not imply that all of thefeatures and advantages that may be realized with the present inventionshould be or are in any single embodiment of the invention. Rather,language referring to the features and advantages is understood to meanthat a specific feature, advantage, or characteristic described inconnection with an embodiment is included in at least one embodiment ofthe present invention. Thus, discussion of the features and advantages,and similar language, throughout this specification may, but do notnecessarily, refer to the same embodiment.

Furthermore, the described features, advantages, and characteristics ofthe invention may be combined in any suitable manner in one or moreembodiments. One skilled in the relevant art will recognize that theinvention can be practiced without one or more of the specific featuresor advantages of a particular embodiment. In other instances, additionalfeatures and advantages may be recognized in certain embodiments thatmay not be present in all embodiments of the invention.

In certain aspects of the present invention, there are providedcomposite solid state CO₂ sensors that are composed of O-MWCNTs andiron-oxide (Fe₂O₃) nanoparticles. The composite solid state CO₂ sensorof the present invention has high CO₂ sensitivity and can measure in therange of about 100 ppm-10000 ppm CO₂. The maximum permissible CO₂exposure limit is 5,000 ppm which has been set by OSHA. The compositesolid state CO₂ sensor of the present invention exhibits up to aboutfive (5) times higher sensitivity to carbon dioxide than conventionalMWCNT-based sensors that do not utilize any iron-oxide particles. Thecomposite solid state CO₂ sensor of the present embodiments has a rapidresponse time and provides sensor outputs in seconds. The compositesolid state CO₂ sensor of the present embodiments also exhibits rapidrecovery, decreased noise and reproducibility of responses.

Carbon nanotubes (CNTs) have a structure allowing the entire surface ofthe carbon nanotube to be available for gas adsorption. As a result, thecarbon nanotube exhibits high sensitivity, down to <1 ppb, and can reactto a single foreign molecule. The electrical characteristics of carbonnanotubes are heavily influenced by gases that donate or acceptelectrons. CO₂ is a weak oxidizing gas. Thus, once CO₂ interacts withthe carbon nanotubes, the CO₂ will take electrons from the carbonnanotubes. The loss of electrons from the carbon nanotubes is reflectedin a measurable resistance change of the carbon nanotubes. Carbonnanotubes have a very high surface area to volume ratio.

Referring to FIGS. 1A and 1B, solid state CO₂ sensor 10 of the presentinvention utilizes sensing material 12 formed with functionalizedO-MWCNTs. Specifically, sensing material 12 is a composite that iscomposed of O-MWCNTs 14 and iron oxide (Fe₂O₃) nanoparticles 16. Sensingmaterial 12 exhibits enhanced sensitivity as a gas sensing material dueto a reaction that induces a modulation of surface charges. Thisenhanced sensitivity is a result of nano-heterojunction formations atthe interface between the O-MWCNTs and iron oxide (Fe₂O₃) nanoparticles.Specifically, the iron oxide has mainly n-type semiconductorcharacteristics and MWCNTs have p-type semiconductor characteristics.These differences result in two depletion layers formed in such hybridfilms. The first depletion region is located at the iron oxide surfaceand the second one is located at the interface between the iron oxidenanoparticles and the O-MWCNTs. The absorption of the CO₂ moleculeinduces a modulation of surface charges that directly influences theelectronic transfer between the heterojunctions and induces a variationin the resistance of sensing material 12. Thus, sensing material 12functions as a chemiresistor wherein the electrical resistance ofsensing material 12 changes in response to increases in theconcentration of CO₂ within the environment to which sensing material 12is exposed. The presence of the iron oxide in the O-MWCNTs matrix alsointroduces nano-channels which play an important role in gas diffusion.The nano-channels allow the gas molecules to easily transport into thegas-sensing layers, leading to increasing CO₂ sensitivity.

FIG. 1A shows a cross-sectional view of a solid state CO₂ sensor inaccordance with some embodiments of the present invention. Sensingmaterial 12, which was described in the foregoing description, comprisesO-MWCNTs 14 and iron oxide (Fe₂O₃) nanoparticles 16, which areillustrated in FIG. 1B.

Further with reference to FIG. 1A, CO₂ sensor 10 further comprisessubstrate 18 and electrodes 20 and 21, which are also shown in FIG. 1C.In an exemplary embodiment, substrate 18 is configured as a grade FR-4printed circuit board (PCB). In some embodiments, substrate 18 is arigid ceramic or silicon material. In an exemplary embodiment,electrodes 20 and 21 are configured as interdigitated electrodes and aremicro-fabricated via screen printing on the surface of substrate 18.However, it is to be understood that any known suitable technique may beused to form electrodes 20 and 21 on substrate 18. For example,electrodes 20 and 21 may be formed on substrate 18 via gravure printing,flexographic printing, ink jet printing, aerosol jet printing, sprayingand slot dye printing. Electrodes 20 and 21 may be formed from anysuitable metals including, but not limited to, gold (Au), palladium(Pd), copper (Cu) and silver (Ag) and platinum (Pt). In an exampleembodiment, electrodes 20 and 21 are formed with gold (Au).

Further in FIG. 1C, solid state CO₂ sensor 10 includes electricallyconductive output terminals or pins 22 and 24 that are electricallycoupled to electrodes 20 and 21, respectively, and are adapted to becoupled to a circuit that can detect and measure changes in theelectrical resistance of sensing material 12.

In an example embodiment, shown in FIG. 1C, output terminals 22 and 24are electrically coupled to the series circuit formed by electricalpower source 26 and electrical current measuring device 28. Electricalcurrent measuring device 28 measures the electrical current passingthrough electrodes 20 and 21 as the concentration of CO₂ changes. Insome embodiments, electrical current measuring device 28 is configuredas a current meter. However, in other embodiments, electrical currentmeasuring device 28 may be configured as any device or electroniccircuit that can detect and measure the flow electrical current throughelectrodes 20 and 21. In some embodiments, power source 26 and currentmeasuring device 28 are part of solid state CO₂ sensor 10. In otherembodiments, power source 26 and current measuring device 28 are part ofexisting electronic circuitry or hardware with which solid state CO₂sensor 10 is integrated. Since sensing material 12 functions as achemiresistor, the electrical resistance of sensing material 12, whichis measured via output terminals 22 and 24, changes in response tochanges in the concentration of CO₂. Due to its unique structure andmaterial composition, solid state CO₂ sensor 10 operates at roomtemperature and exhibits fast response times to concentrations of CO₂ranging from 100 ppm to 0.5% CO₂ (OSHA maximum permissible exposurelimit).

With reference to flow chart FIG. 1D, an example process 100 offabricating the solid state CO₂ sensor of the present invention isproviding substrate 18 at step 101. At step 103, electrodes 20 and 21are formed on substrate 18 via any of the techniques described in theforegoing description. At step 107, O-MWCNTs are formed by providing anamount of MWCNTs, and then mixing the MWCNTs with a mixture of acids,then heating the mixture to oxidize the MWCNTs. In an exampleembodiment, the mixture of acids comprises concentrated sulfuric acid(98% H₂SO₄) and nitric acid (68% HNO₃) with the volume ratio of 3:1. Inan example embodiment, oxidation is completed by exposure to atemperature at about 120° C. for a period of about 2 hours. At step 109,dilution, decantation and centrifugation are performed on the O-MWCNTs.If necessary, the dilution, decantation and centrifugation steps may berepeated. At step 111, the O-MWCNTs are then rinsed in water, and thepurified O-MWCNTs are then dried at a predetermined temperature for apredetermined amount of time. In an example embodiment, the purifiedO-MWCNTs are dried at about 125° C. for about 3.0 hours using aprogrammable oven.

At step 113, the dry powder of the O-MWCNTs is then mixed with apredetermined weight percentage of iron oxide (Fe₂O₃) nanoparticles inwater to make a uniform suspension. In some embodiments, the weightpercentage of iron oxide (Fe₂O₃) in the uniform suspension is betweenabout 3.25% and 3.75%. In an exemplary embodiment, the weight percentageof iron oxide (Fe₂O₃) in the uniform suspension is about 3.5%. It hasbeen found that a weight percentage of about 3.5% iron oxide (Fe₂O₃)provides the optimal sensor performance for the detection of CO₂. It hasfurther been found that the O-MWCNTs produce significantly more uniformsuspension in water in comparison to non-oxidized MWCNTs. At this point,the uniform suspension is the liquid form of sensing material 12described in the foregoing description.

At step 115, the uniform suspension of 3.5% iron oxide and O-MWCNTs inwater is deposited via a micropipette onto interdigitated electrodes 20and 21 and onto the gaps or areas between interdigitated electrodes 20and 21. At step 117, the raw CO₂ sensor (i.e., substrate 18 withelectrodes 20 and 21 covered with uniform suspension) is subjected todehydration baking in a vacuum oven in order to heat and dry any wateroff the surface and leave a dry O-MWCNTs/iron oxide (Fe₂O₃) compositesensing material on the surface of interdigitated electrodes 20 and 21and the gap areas between interdigitated electrodes 20 and 21. In anexemplary embodiment, the dehydration baking temperature is betweenabout 200° C. and 400° C. and the dehydration baking time is betweenabout 30-60 minutes. The dry O-MWCNTs/iron oxide (Fe₂O₃) composite layerfunctions as sensing material 12 of solid state CO₂ sensor 10 and isbonded to electrodes 20 and 21 and the gap areas between electrodes 20and 21 via van der Waals force due to the large surface area and lightweight of the carbon nanotubes.

CO₂ gas exposure tests were conducted on CO₂ sensor 10 using the testingconfiguration shown in FIG. 1E. Output terminals 22 and 24 of CO₂ sensor10 were electrically coupled to interface board 30. For this test,interface board 30 was a low-current scanner card. Interface board 30 iselectrically coupled to a Keithley Series 2700 Multimeter/DataAcquisition/Switch System 32 which was set up to measure the electricalresistance of CO₂ sensor 10 during exposure to different amounts of CO₂gas. The Keithley Series 2700 Multimeter/Data Acquisition/Switch Systemis manufactured by Keithley Instruments of Scottsdale, Ariz. In order toexpose CO₂ sensor 10 to the CO₂ gas, Teflon cover 34 was positioned overCO₂ sensor 10. A nozzle (not shown) was attached to the interior ofTeflon cover 34. The nozzle was adapted to introduce the stream of CO₂gas directly onto the surface of CO₂ sensor 10. An Environics Gas MixingSystem (Environics Inc., Tolland, Conn.) 44 was connected CO₂ gas source40, dry air source 42 and to the nozzle (not shown) that is connected toTeflon cover 34. The CO₂ gas source 40 provided 10000 ppm CO₂. The dryair source 42 provided dry Zero Air. The Environics Gas Mixing System isa gas blending and dilution system and was used for introducing CO₂ gasat different concentrations to the nozzle within Teflon cover 34. Priorto exposing CO₂ sensor 10 to CO₂ gas, a stream of dry Zero Air wasdirected onto CO₂ sensor 10 for ten (10) minutes in order to establish abaseline resistance for the following CO₂ exposure and dry air flushcycles. Next, the Environics Gas Mixing System was activated so as toprovide a gas stream of 400 cm³/min to the nozzle inside Teflon cover 34so as to expose CO₂ sensor 10 to CO₂ gas. CO₂ concentrations were runfor a minute followed by a five (5) minute dry Zero Air flush. CO₂concentrations were increased from 100 ppm to 6000 ppm. The tests wereconducted at room temperature. FIG. 2 shows a plot of responses of CO₂sensor 10 to the increasing concentrations of CO₂ gas. CO₂ sensor 10exhibited high sensitivity. CO₂ sensor 10 also exhibited a stable baseresistance of about 13000 Ohms thereby indicating CO₂ sensor 10 can besuccessfully integrated into existing electronics circuitry or hardwaresystems. Thus, CO₂ sensor 10 exhibited strong responses for highconcentrations of CO₂ gas. CO₂ sensor 10 also exhibited low noise, quickresponse and quick recovery times as well as high repeatability orreproducibility of responses.

The morphology of the O-MWCNT, iron oxide (Fe₂O₃) nanoparticles and thenanocomposites of O-MWCNT/iron oxide (Fe₂O₃) that form sensing material12 were investigated using a FE-SEM Hitachi S-4800 SEM. It has beenfound that the iron oxide (Fe₂O₃) nanoparticles formed chemical bondswith the oxidized MWCNT surface. Specifically, and as shown in FIG. 3 ,it can be seen that the MWCNTs form bundles due to the strong Van derWaals force. MWCNT diameters ranged from about 4 nm to about 10 nm. Themorphology of the iron oxide image presents clusters of the iron oxidesthat cling together and form aggregates in spherical shapes withindividual sizes ranging from about 5 nm to about 50 nm. The clusters ofthe iron oxide result from the magneto static coupling betweenparticles. The SEM images of the O-MWCNT/iron oxide nanocomposites shownetworks of MWCNTs interwoven among the iron oxide nanoparticles. Theiron oxide particles adhered on the surface of O-MWCNTs and formed smallagglomerations as free particles. The iron oxide is distributeduniformly on the surface of the MWCNTs.

The testing system shown in FIG. 1E was used to measure the electricalresistance of CO₂ sensor 10 and an O-MWCNT-based sensor without ironoxide (Fe₂O₃) nanoparticles. The O-MWCNT-based sensor without iron oxide(Fe₂O₃) nanoparticles had a resistance of about 1300 ohms. CO₂ sensor 10had an average base resistance of about 13000 Ohms. The testing systemshown in FIG. 1D was used to compare responses of CO₂ sensor 10 and anO-MWCNT-based sensor without iron oxide (Fe₂O₃) nanoparticles. Bothsensors were exposed, at room temperature, to CO₂ concentrations of 100,200, 400, 800, 1600, 3800 and 6000 ppm at the same time intervals usedto test CO₂ sensor 10 with the testing system of FIG. 1E as described inthe foregoing description. FIG. 4 shows the response curves for CO₂sensor 10 and the oxidized MWCNT-based sensor without (Fe₂O₃)nanoparticles. The response is normalized resistance ΔR/R₀ wherein R₀ isthe base electrical resistance with no CO₂ exposure and ΔR=R−R₀ whereinR is the electrical resistance at time “t” with CO₂ exposure. Sensorresistance increased when exposed to various concentrations of CO₂ gasand the change in sensor resistance was concentration dependent. CO₂sensor 10 exhibited a response that was about five (5) times greaterthan the O-MWCNT-based sensor without (Fe₂O₃) nanoparticles. This is theresult of the CO₂ molecules being absorbed at both the MWCNT surface andthe iron oxide nanoparticles. CO₂ sensor 10 exhibited a greater responserange in comparison to the O-MWCNT-based sensor without (Fe₂O₃)nanoparticles. FIG. 4 also shows that CO₂ sensor 10 exhibited enhancedresponses at CO₂ concentrations of 3800 ppm and 6000 ppm while theO-MWCNT-based sensor without (Fe₂O₃) nanoparticles did not show distinctpeaks for CO₂ concentrations of 3800 ppm and 6000 ppm.

Referring to FIG. 5 , there is shown a plot of the responses of CO₂sensor 10 for multiple exposures of CO₂ at a concentration of 4000 ppm.The responses of CO₂ sensor 10 were all between about 0.0325 and about0.0385.

The solid state CO₂ sensor of the present invention provides manybenefits and advantages. For example, the CO₂ sensor has a rapidresponse time and can provide sensor outputs in seconds. The CO₂ sensorhas high CO₂ sensitivity and can measure CO₂ having a concentrationwithin the range of about 100 ppm and 10000 ppm. It has been found thatthe CO₂ sensor of the present invention has up to about five (5) timeshigher sensitivity to CO₂ than a MWCNT-based sensor without iron-oxideparticles. The CO₂ sensor of the present also exhibits rapid recovery,decreased noise and reproducibility of responses. The CO₂ sensor of thepresent invention operates at room temperature, 25° Celsius, unlikeconventional metal oxide sensors which must operate at ˜300° Celsius.The CO₂ sensor of the present embodiments also operates in high humidityconditions (>80% RH) and in static diffusion mode conditions and dynamicflow mode conditions (e.g., 400 ccm). The CO₂ sensor of the presentinvention has a small footprint. Specifically, CO₂ sensor 10 can have arelatively small size of 0.5 cm×0.5 cm×3 mm with multiple sensors formedon substrate 18. Such small sensor size is in stark contrast to thesmallest sized conventional NDIR sensor which is 2.5 cm.×1.25 cm.×1.25cm. The solid state CO₂ sensor of the present invention is light inweight, weighting only a few grams. Another important benefit of thesolid state CO₂ sensor of the present invention is that it consumesrelatively less power. Specifically, less than fifty (50) microwatts arerequired for power since CO₂ sensor 10 functions on changes inresistivity. On the other hand, many conventional sensors function onoptical properties or changes in capacitance. Auxiliary electricaloutput terminals 22 and 24 are built into solid state CO₂ sensor 10 andallow CO₂ sensor 10 to be easily integrated or coupled into existingelectronic circuitry. Since the CO₂ sensor 10 is solid state, noconsumables or extra materials are required to maintain operation. Thesolid state CO₂ sensor 10 does not require calibration as often asconventional CO₂ sensors and the calibration of the CO₂ sensor 10 can beautomated. Another significant benefit is that per/unit manufacturingcosts of the solid state CO₂ sensor is significantly less than per/unitmanufacturing costs of many conventional CO₂ sensors. Anothersignificant benefit is that due to the homogenous nature of the sensingmaterial, the sensor-to-sensor variation is negligible thereby makingthe sensor suitable for large scale commercial production.

In some embodiments of the present invention, CO₂ sensor 10 isintegrated into a smart phone to detect CO₂ gases in real time. One suchan embodiment is shown in FIG. 6 . Smart phone 50 comprises casing 52,display screen 54, home button 56 and speaker port 58, all of which areknown in the smart phone industry. Internal electronic circuitry 60 insmart phone 50 is programmable and includes data acquisition circuitrythat is coupled to electrically conductive output terminals 22 and 24 ofCO₂ sensor 10 and is configured for detecting and measuring theelectrical current flow through the electrodes 20 and 21. Smart phone 50is programmed with an APP (i.e. mobile App) that determines if thedetected current flow represents a concentration of CO₂ that exceeds apredetermined concentration of CO₂ and alerts a user if it is determinedthat the concentration of CO₂ exceeds the predetermined concentration ofCO₂. In an exemplary embodiment, if the measured concentration of carbondioxide exceeds the predetermined concentration, internal electroniccircuitry 60 controls display screen 54 to display a message informingthe user of the concentration of CO₂. In another exemplary embodiment,if the concentration of CO₂ exceeds a predetermined or preset level,then internal electronic circuitry 60 generates an audio alarm. In someembodiments, the audio alarm will continue until the user deactivatesthe audio alarm by touching the appropriate icon on display screen 54.Internal electronic circuitry 60 includes additional data processingcircuitry and data storage circuitry that also execute and implementnormal or typical smart phone functions. Integrating CO₂ sensor 10 intosmart phone 50 provides many advantages, including a low cost signalprocessing platform, compactness, low power consumption, easy operation,rapid sensing and network sensing capability.

The CO₂ sensor of the present invention may be used to detect lowconcentrations of CO₂ at room temperature and with low powerconsumption. There are many space applications in which the CO₂ sensorof the present invention may be used. For example, the CO₂ sensor of thepresent invention may be placed or positioned in the crew cabin of theInternational Space Station (ISS) in order to continuously monitorreal-time CO₂ concentrations for permissible exposure limit levels. Ifthe real-time CO₂ concentrations exceed the permissible exposure limitlevels, then the space life support system would be deployed to scrubthe station atmosphere. In another space application, the CO₂ sensor ofthe present invention can be used to determine the catalyzing efficiencyof the CO₂ splitting process on the planet Mars. This process is knownas in-situ resource utilization and is necessary to produce breathingoxygen for astronauts. In some embodiments, the CO₂ sensor of thepresent invention is configured as a wearable sensor. In one such anembodiment, CO₂ sensor 10 is integrated with an RFID (radio frequencyidentification) platform so as to enable the wearable sensor to be usedfor astronaut atmospheric monitoring inside the astronaut's helmet. TheCO₂ sensor of the present invention may be configured as a miniature“Lick and Stick” smart CO₂ sensor.

The foregoing description, for purpose of explanation, has beendescribed with reference to specific embodiments. However, theillustrative discussions above are not intended to be exhaustive or tolimit the invention to the precise forms disclosed. Many modificationsand variations are possible in view of the above teachings. Theembodiments were chosen and described in order to best explain theprinciples of the invention and its practical applications, to therebyenable others skilled in the art to best utilize the invention andvarious embodiments with various modifications as are suited to theparticular use contemplated.

What is claimed is:
 1. A method of fabricating a sensor for sensingcarbon dioxide (CO₂), comprising: providing a substrate; disposing apair of electrodes on the substrate; providing a predetermined amount ofmulti-walled carbon nanotubes (MWCNTs); oxidizing the MWCNTs with amixture of acids at a first predetermined temperature and for a firstpredetermined amount of time; diluting, decanting and centrifuging theoxidized multi-walled carbon nanotubes (O-MWCNTs); rinsing the O-MWCNTsin water to produce purified O-MWCNTs; drying the purified O-MWCNTs at asecond predetermined temperature for a second predetermined amount oftime to produce purified O-MWCNTs in dry powder form; forming a solutionconsisting of water, the purified O-MWCNTs in dry powder form and ironoxide (Fe₂O₃) nanoparticles so as to produce a uniform suspension;depositing the uniform suspension onto the electrodes; and thereafter,baking the substrate so as to transform the uniform suspension into adry (CO₂) sensing material.
 2. The method according to claim 1 whereinthe step of disposing a pair of electrodes on the substrate comprisesforming a pair of interdigitated electrodes on the substrate.
 3. Themethod according to claim 1 wherein the mixture of acids comprisessulfuric acid and nitric acid.
 4. The method according to claim 1wherein the weight percentage of iron oxide (Fe2O3) nanoparticles in thesolution is between 3.25% and 3.75%.
 5. A sensor made according to themethod of claim 1.